(1) Field of the Invention
This invention relates generally to Doppler processing and more particularly to a method of extracting target range and Doppler information about velocity of the target from a Doppler-spread active sonar echo.
(2) Description of the Prior Art
Acoustic signal processing techniques using the Doppler shift to extract target range data are well known in the art. It is also well known that the time-varying attenuation of a target causes a Doppler-spread energy spectrum in the frequency domain. This phenomenon is taught by Harry L. Van Trees in "Detection, Estimation and Modulation Theory, Part III", John Wiley and Sons, Inc., 1971, a brief overview of which follows hereinbelow to facilitate understanding of the present invention.
An active sonar system, basically, transmits an acoustic signal into the ocean and from the returned echo attempts to extract information about the target. Its performance depends to a large extent on the motion of the target and ocean characteristics such as its non-linearity. Often, the ability of the sonar system to extract range and Doppler information is degraded by a phenomenon called Doppler spreading, which can be caused by the target and/or by the medium. This phenomenon also arises in radar, communications, and optical applications. A transmitted sonar signal can be Doppler-spread from:
a) The changing orientation of the target during the time that the transmitted signal interacts with it. Physically, this is characterized by the pulse length being longer than the reciprocal of the target reflection process.
b) The propeller on stern aspect targets. A similar effect is observed from radar returns of proller-driven aircraft.
c) The interference from scatterers fo the target. Typical sonar returns are shown in R. Urick, Principles of Underwater Sound, McGraw-Hill, Inc., 3rd Edition, 1983, on page 325.
d) The fluctuations caused by the medium. For the sonar application fluctuations would have to occur over the pulse duration. These are typically characterized as fast fading.
e) The physical effects causing platform motion and vibration.
A target geometry 10 representative of any reflective surface such as an airplane, a satellite or a submarine is shown in FIGS. 1(a), (b) and (c). The direction of signal propagation is along the x-axis. The target orientation in FIGS. 1(a), (b) and (c) changes as a function of time where FIG. 1(a) is at time t.sub.1, FIG. 1(b) is at time t.sub.2 and FIG. 1(c) is at time t.sub.3. As the orientation of target geometry 10 changes, so do its reflective characteristics.
The target geometry 10 is illuminated with a long acoustic pulse f(t), t=0 to T.sub.L where T.sub.L &gt;t.sub.3, as shown in FIG. 2(a). A typical return signal envelope s(t) as a function of time might look like that shown in FIG. 2(b). It is readily apparent that the effect of the changing orientation of target geometry 10 is a time-varying attenuation of the envelope. Since the time-varying attenuation is an amplitude modulation, the energy spectrum of the return signal E[s(j.omega.)] is spread in the frequency domain as shown in FIG. 2(c). The amount of spreading depends on the rate at which the target geometry's reflective characteristics are changing. This type of target is known as a Doppler-spread target.
Current acoustic processing techniques make use of a second order spectrum to extract range and Doppler information from a return signal envelope. However, the prior art method is not effective in extracting such information when the target is a Doppler-spread target undergoing orientation changes. The prior art method is also susceptible to additive Gaussian noise which degrades the signal-to-noise ratio.